With respect to a color image forming apparatus, it has previously been conceived to produce patch marks in respective colors on a carrier element (such as a conveying belt) that rotates in synchronism with an operation of an image forming unit (such as a process cartridge), and to correct image density of respective colors by detecting densities of the patch marks. In an image forming apparatus of this type, the densities of the patch marks are calculated by individually detecting specular reflected light and diffuse reflected light from the patch marks. Further, correcting a detected value of diffuse reflected light in accordance with a ratio between specular reflected light and diffuse reflected light reflected by an area having high density.
However, when a correction coefficient is calculated by using patch marks of one density type made in a high density area, there will arise a situation where the influence of stray light, and the like, cannot be eliminated as described below. For example, in the case of a structure having a light emitting element for irradiating a carrier element with light, a first light receiving element for detecting the specular reflected light, and a second light receiving element for detecting the diffuse reflected light, each fixedly inserted into a resin member respectively, the back sides of each of the light emitting element and the light receiving elements are opened, and light can sometimes escape toward the rear of the back sides (i.e., the direction opposite to the carrier element). When some of the escaped light, (also-called stray light) enters any of the light receiving elements, a received light output will occur even when incident light originating from the carrier element is zero. In such a case, values detected by the light receiving elements are shifted on the whole. Thus, it is impossible to sufficiently eliminate the influence of stray light by using only calculating a correction coefficient based on the ratio between specular reflected light and diffuse reflected light from the patch marks of one density type. Moreover, a component of stray light is dependent on the quantity of light emitted by an irradiation unit. Therefore, when the quantity of emitted light is increased by any one of a change in a use environment, a change in an apparatus with time, and the like, the error in the measurements will increase.
Accordingly, an exemplary embodiment of the present invention may make it possible for an image forming apparatus, which produces chromatic color patch marks on a carrier element and detects specular reflected light and diffuse reflected light from the patch marks, to eliminate the influence of stray light from detected values of the specular reflected light and the diffuse reflected light and to thereby accurately detect a density. However, embodiments of the present invention need not solve this or any other problems.